Nutrition: - its final Cause. - Old Age: - Sir A. Carlisle's View of the Disorders of Senility objectionable. - The Author's opinion. - Chemical and Mechanical Agents of Digestion. - Conventional acceptation of the term Chemical. - Mastication. - Insalivation. - Erroneous Opinion of Dr. Fordyce and others with respect to the Use of the Saliva. - Deglutition. - Action of the Gastric Juice. - Chymification: - Artificial Digestion. - Office of the Pylorus.- - Vomiting. - Duodenal Digestion. - Chylification: - Uses of the Bile. - Sir B. Brodie's Experiments. - Chemical Nature of the Chyle: - its Absorption. - Excrementitious Matter. - Liquid Aliments: - how decomposed in the Stomach. - The liquid part absorbed or coagulated. - Digestion of Milk, Broth, Wine, Oil, etc. - Lymphatic Absorption. - Sanguification. - Respiration. - Practical Conclusions. - Urinary Secretion. - Acidifying Powers of the Kidney. - The Quality of Urine influenced by the Digestive Process. - Necessity of Urinary Secretion.- - A general View of the Digestive Process. - Reasons for believing in the existence of an Agent analogous to Electricity. - Experiments of Dr. Wilson Philip. - Speculations of the Author.

67. A very superficial examination of an organized body will convince us, that it is constantly losing portions of the matter of which it is composed: several of its organs are incessantly engaged in separating fluids, which are loaded with its more solid constituents; and it is on the necessity of repairing these habitual losses, that the want of aliment is founded; while the assimilation of such nutritive materials with the composition of the organs which they are destined to supply, constitutes nutrition. And with such nicety are these processes of waste and repair adjusted, that, whatever may be the quantity of food taken, or however the circumstances under which it is consumed may vary, the same individual, after having augmented in weight in proportion to the quantity of ingesta, will return, in the space of twenty-four hours, to nearly the same standard, provided he is not growing, nor has suffered any disorder of function. During this period the food has been decomposed, and re-combined into compounds analogous to those which compose the organs to which it is carried; and this appears to take place with the same facility, however remote the composition of the aliment may be from that of the substances with which it assimilated.

Living bodies, then, have not inaptly been compared to furnaces, into which inert substances are successively thrown, which combine amongst themselves in various manners, maintain a certain place, and perform an action determined by the nature of the combinations they have formed, and at last fly off, in order to become again subject to the laws of inanimate nature. These views naturally suggest several important questions for our consideration. If every part of the machine be thus capable of immediate and constant repair, why should it ever wear out? Does there exist some secret spring which is incapable of renewal, and at whose expense all the subordinate parts of the machinery are kept in repair? And why should life be terminated by the hardening of fibres, and the obstruction of vessels, when those very fibres and vessels are susceptihle of renewal? These are mysteries which cannot be cleared up until the sealed fountain of vitality be laid open. It seems probable, that every individual has a certain measure of living energy assigned to him, which is gradually expended in directing and maintaining the performance of certain functions: when this is exhausted, the individual must perish, for it is incapable of renewal.

Sir A. Carlisle, in his work on the Disorders of Old Age, remarks, that it seems little better than a vulgar error, to consider the termination of advanced life as the inevitable consequence of time, when the immediate cause of death in old persons is generally known to be some well-marked disease. I have directed some attention to this subject, and I feel warranted by experience to state, that the greater number of those who terminate their existence at an advanced age, die from the exhaustion of vital power, and that this is manifested in some one of their principal organs, the consequence of which is an ill-marked species of inflammation. Symptoms indicating the existence of peritonitis are by no means uncommon; in some cases, the lungs or the brain appear to be the seats of disease. The arterial system owes its regularity of action to the presiding influence of the nervous power; and if this be withdrawn, or irregularly supplied, inflammation follows. It is to such a cause that the local congestions, and topical inflammations, which so frequently occur in fever, are to be attributed.

With all due respect, therefore, for the experience and skill of the author to whom I allude, I must differ with him in the view he has taken of the disorders of senility: the doctrine he inculcates is dangerous, and may lead to practical mischief. In cases of congestion, blood-letting is undoubtedly a judicious remedy, when directed with measured caution, because it removes an effect which may contribute to aggravate the original disease, or to obstruct the sanative operations which nature institutes for her own relief: but we should perfectly understand the mode in which it operates, and not mistake secondary for primary diseases.

66. Although these views may disparage every effort to prolong the natural term of existence, they afford us the satisfaction of knowing that we may ward off those accidents which would otherwise lead to its premature termination. Although we cannot augment the allotted measure of our vital energy, we can, at least, learn to husband its resources, and not to consume, with wanton indifference, the unrecruitable oil by which the lamp of life is supported.

68. In examining the phenomena of waste and supply, we shall observe that there is a marked difference, depending on age, health, temperament, and bodily exercise, in the proportion of the parts which enter into this current, and of those which abandon it; and that the velocity of the motions usually varies according to the different conditions of each living being. A knowledge of these differences, which is to be discovered only by ample experience and well-directed observation, must constitute the basis of a true theory, for the regulation of diet. If it be said, that a deficient quantity of food is indicated by our feelings, and that an excess is carried off without inconvenience, I shall reply, that, under ordinary circumstances, nature rarely suffers from abstinence, but continually from repletion; that while, in one case, she limits her expenditure to meet the exigencies of her income, in the other, she is called upon to exercise an injurious liberality to throw off by various secretions and exhalations the useless burden. In the vigour of health and youth, the evils of such a system may not be felt, but as life advances, organic changes are slowly and imperceptibly produced, which ultimately, by their aggregate effect, clog the machine and lead to disease and premature death.

No one has illustrated this subject more forcibly and philosophically than Mr. Travers in a late work, to which I shall have occasion to refer more fully on a future occasion.